The invention relates to the production of oxidic nanoparticles from a material that forms oxide articles, in particular metal-oxide particles, and the formation of a film of such particles on any substrate.
Nanostructures that are themselves nanoparticles or are formed by a uniform distribution and/or regular arrangement of nanoparticles on a substrate, are presently at the centre of research. They represent a class of materials that exhibit novel electrical, optical, magnetic, and thermodynamic properties among others due to quantum effects. In addition to the primarily academic issues, the problem of reproducible mass production with means as simple as possible arises in the context, so that the field of commercial utilization of nanostructures can be achieved in a short period of time.
Regarding the general state of the art concerning the manufacture of nanostructures, the following printed publications are referred to: D. Rovillain et al.: “Film boiling chemical vapor infiltration—An experimental study on carbon/carbon composite materials”, Carbon 39, pp. 1355-1365 (2001); B. J. Urban and C T. Avedisian, W. Tsang: “The Film Boiling Reactor: A New Environment for Chemical Processing”, AIChE J. 52(7), pp. 2582-2595 (2006); DE 10 2005 060 407 B3; DE 103 92 447 T5; Dongsheng Wen and Yulong Ding: “Experimental investigation into the pool boiling heat transfer of aqueous based y-alumina nanofluids”, Journal of Nanoparticle Research 7, pp. 265-274 (2005) and S. M. You, J. H. Kim, K. H. Kim: “Effect of nano-particles on critical heat flux of water in pool boiling heat transfer”, Appl. Phys. Lett. 83(16), pp. 2274-2276 (2003).
In particular the production of a uniformly distributed film of nanoparticles on the surface of a substrate has until today been a difficult process that is usually associated with several steps and high costs, in particular if the nanoparticles are furthermore to be arranged in a specific manner. Typical methods for the production of such structures are Vapor Liquid Solid (VLS) or MOCV methods. Although these methods can be employed relatively universally, however both the control of the atmosphere (UHV) and also the necessity for high temperatures (600-1000° C.) require expensive equipment and make the synthesis time-consuming. Pre-structured substrates as for example MEMS cannot be exposed to such high temperatures just like that.
To reduce the outlay, the person skilled in the art has knowledge also of wet chemical manufacturing methods from an aqueous solution that lead to the desired results at temperatures below 100° C. and at atmospheric pressure (for example see Law et al., “Nanowire dye-sensitized solar cells, Nature Materials, 4, 455-459, 2005). However, the wet chemical methods have other disadvantages in addition to proceeding very slowly (process times of several hours up to days). For example, no epitaxial growth on silicon is possible (see J. Phys. Chem. B 2001, 105, 3350-3352). Also in some cases solvents are used that could lead to disposal problems.
Today the interest is very high regarding the manufacture of zinc oxide (ZnO) nanostructures such as nanorods and nanotubes. This is the case in particular on account of the fact that ZnO as a semiconductor can form a great variation in terms of nanostructures. On top of this, versatile applications such as opto-electronic components, lasers, field emission and gas sensor materials are envisaged (for the manufacture and application of nanotubes and nanorods see also Advanced Materials 2005, 17, 2477). So that ZnO structures can be produced epitaxially, either special substrates such as gallium nitride (GaN) are used or silicon substrates are coated with a so-called “seeding layer” that usually consists of a ZnO thin film heated to 400° C.
The patent application DE 10 2005 060 407 A1 reveals a direct, non-epitaxial production of ZnO structures distributed over the surface of substrates. In the process, metal salts, in particular zinc acetate, are dissolved in water and made to drip on a previously heated substrate (for example a hot plate). The drops evaporate, floating on the steam cushion (Leidenfrost effect), and distributed nanostructures remain on the wetted substrate surface, in particular for example small tufts of ZnO wires.
The Leidenfrost effect seems essential for the formation of the nanoparticles from the metal ions that have previously been dissolved in water. For this effect, a very fast reaction kinetics in non-equilibrium at the phase boundary fluid/steam at the bottom side of the drop is assumed to be the cause. The precise nature of the processes there has so far not been explained.
Apart from further advantages for example in terms of the directional arrangement of nanowires, the method described in DE 10 2005 060 407 A1, however, also exhibits the disadvantage that it is not suited just like that for coating any substrates. Problems seem to occur with curved, locally recessed or above all angled-off substrate surfaces if the vaporizing drops are to float across. Furthermore, the substrate must be able to withstand temperatures above 200° C. for longer periods of time—it is not to oxidize in the process, which limits the choice of material among others in the field of plastics.
It has, however, been pointed out in a publication that appeared recently (Urban and Avedisian, “The Film Boiling Reactor: A New Environment for Chemical Processing”, AIChE Journal, 52, 7, pp. 2582-2595 (2006)), that the process of film boiling, that resembles the Leidenfrost effect, makes effective chemical conversions possible. Specifically a so-called FIBOR (“film boiling reactor”) for producing hydrogen gas from methanol is described there.